1. Field of the Invention
The present invention relates to a high-purity calcium hydroxide having fine particles and a high dispersibility in a resin, a process for the production thereof and a calcium hydroxide-containing resin composition containing said calcium hydroxide as an active ingredient for an acid-capturing agent. More specifically, it relates to calcium hydroxide which can work as a heat stabilizer for a resin, prevent the occurrence of dioxin or the like, prevent the corrosion of a processing machine or impart antifungal properties owing to the capture of an acid substance such as hydrochloric acid, generated from a resin or garbage at a processing or burning time, at high yields by incorporating a high-purity calcium hydroxide having fine particles and a high dispersibility, as an active ingredient, into a resin such as polyvinyl chloride or polyethylene, a process for the production thereof and a calcium hydroxide-containing resin composition containing said calcium hydroxide as an active ingredient for an acid-capturing agent.
2. Description of the Prior Art
A conventional calcium hydroxide has large primary particles (crystal) as large as about 1 xcexcm to several xcexcm, and these particles agglomerate strongly to form large secondary particles (the average secondary particle diameter at a cumulative percentage of 50% is about 4 to 20 xcexcm). Further, the above calcium hydroxide is very unstable, and it has the problem that the calcium hydroxide is easily reacted with a carbonic acid gas in the air to convert to calcium carbonate. When the conventional calcium hydroxide is added in a resin, therefore, the calcium hydroxide is remarkably poor in dispersibility in the resin. For example, when a resin composition containing the calcium hydroxide is molded to a film, the surface of the film is rough, and further, the film is colored to have a puce color due to large amounts of impurities such as Fe or Mn. Further, the reactivity of the calcium hydroxide with an acid such as hydrogen chloride generated from polyvinyl chloride is impaired since the primary particles and the secondary particles are large. Therefore, a corresponding amount of the calcium hydroxide should be further added to the resin. When the amount of the calcium hydroxide increases, inherent properties of a resin are impaired, which extremely decreases a value as a commodity product.
It is an object of the present invention to provide a fine-particle, high-dispersibility and high-purity calcium hydroxide of which the primary particles and secondary particles are fine, and which is prevented from converting to calcium carbonate, and a process for the production thereof.
It is another object of the present invention to provide a calcium hydroxide (acid-capturing agent)-containing resin composition which has a high dispersibility, a high purity and high whiteness and which is excellent in the appearance of a molded article, mechanical strength, thermal stability, antifungal properties and acid-capturing properties.
According to the present invention, there is provided a calcium hydroxide in which the average secondary particle diameter at a cumulative percentage of 50% by number in a particle size distribution is 2.0 xcexcm or less, preferably 0.1 to 1.5 xcexcm, particularly preferably 0.5 to 1.1 xcexcm, and the BET specific surface area is 7 to 20 m2/g, preferably 8 to 20 m2/g, more preferably 9 to 18 m2/g, particularly preferably 10 to 15 m2/g. According to the present invention, there is provided a calcium hydroxide which is surface-treated with 0.1 to 10% by weight, preferably 0.5 to 5% by weight, more preferably 1 to 5% by weight, of an anionic surfactant.
According to the present invention, further, there is provided a calcium hydroxide-containing resin composition obtained by incorporating 0.1 to 100 parts by weight, preferably 0.2 to 50 parts by weight, particularly preferably 0.5 to 20 parts by weight, of said calcium hydroxide as an active ingredient into 100 parts by weight of a synthetic resin.
According to the present invention, further, there is provided a process for the production of the calcium hydroxide as recited above, comprising adding a water-soluble calcium salt aqueous solution to an aqueous solution containing at least one equivalent of alkali metal hydroxide to calcium with stirring, allowing the mixture to react at 30 to 90xc2x0 C., preferably 40 to 90xc2x0 C., then aging the resultant mixture at 40 to 120xc2x0 C., preferably 40 to 80xc2x0 C., for preferably about 0.1 to 2 hours to synthesize calcium hydroxide, and then adding an aqueous solution of an anionic surfactant in an amount of 0.1 to 10% by weight based on the calcium hydroxide at a temperature where the anionic surfactant is soluble or at a higher temperature with stirring, to surface-treat the calcium hydroxide.
According to the present invention, further, there is provided a process for the production of the calcium hydroxide as recited above, comprising wet-pulverizing a slaked lime obtained by slaking a quicklime (calcium oxide) having small contents of impurities such as silicone dioxide, alumina and ferric oxide (Fe2O3), then adding an anionic surfactant to the wet-pulverized slaked lime in a water medium with stirring to surface-treat the slaked lime, or adding the anionic surfactant before the wet-pulverization and carrying out the wet-pulverization and the surface-treatment at the same time.
In the calcium hydroxide used in the present invention, the secondary particles of the calcium hydroxide are required to be fine particles, and therefore, the primary particles of the calcium hydroxide are also required to be fine particles. These states of the primary and secondary particles make it possible to give a good appearance of a molded article and to improve various properties such as mechanical strength and high acid-capturing properties.
Concerning the primary particles, at least the average particle diameter thereof is 1 xcexcm or less, preferably 0.5 xcexcm or less. The average particle diameter of the primary particles is measured by a scanning electron microscope (SEM) method. The secondary particles are ultrasonically treated in an isopropyl alcohol solvent for five minutes to be dispersed, and then, the average particle diameter of the secondary particles is determined by a particle size distribution measured by a laser diffraction method. The average secondary particle diameter at a cumulative percentage of 50% by number in the particle size distribution is 2.0 xcexcm or less, preferably 0.1 to 1.5 xcexcm, particularly preferably 0.5 to 1.1 xcexcm. Concerning the secondary particles, besides the above definition in case of the cumulative percentage of 50% by number, the average secondary particle diameter at a cumulative percentage of 90% by number is preferably 8 xcexcm or less, more preferably 5 xcexcm or less, furthermore preferably 4 xcexcm or less, particularly preferably 2 xcexcm or less.
The BET specific surface area almost corresponds to the size of the primary particles. The BET specific surface area of the calcium hydroxide of the present invention is 7 to 20 m2/g, preferably 8 to 20 m2/g, more preferably 9 to 18 m2/g, particularly preferably 10 to 15 m2/g. When the BET specific surface area is larger than 20 m2/g, the calcium hydroxide is likely to agglomerate and the viscosity of a resin becomes too high so that it is difficult to knead a resin composition containing the calcium hydroxide and the moldability of the resin composition is deteriorated. Conversely, when the BET specific surface area is smaller than 7 m2/g, the primary particles become too large so that activities such as acid-capturing properties are decreased.
The calcium hydroxide used in the present invention may be produced by the following two methods. In the first method, the calcium hydroxide can be produced by adding an aqueous solution of a water-soluble calcium salt such as calcium chloride or calcium nitrate into an aqueous solution of alkali metal hydroxide such as sodium hydroxide and potassium hydroxide in an alkali ratio of at least one equivalent, preferably 1.1 to 1.3 equivalent, to calcium at 30 to 90xc2x0 C., preferably 40 to 90xc2x0 C., with stirring, allowing the mixture to react, then aging the reaction mixture under heat at preferably about 40 to 120xc2x0 C., particularly preferably about 40 to 80xc2x0 C., for about 0.1 to 2 hours and adding an anionic surfactant to the aged mixture to carry out a surface-treatment.
The second method is a method of reacting quicklime obtained by the calcination of natural lime having a high purity with water at a temperature of preferably about 60 to 90xc2x0 C. to form slaked lime and, preferably, wet-pulverizing it with a ball-mill. In this case, the diameter of a ball consisting of alumina, zirconia, glass or the like is about 2 mm or less, preferably 0.5 to 2mm, and the use of such a ball is preferred. The period of time for pulverization differs depending upon a kind of a machine, while it is about 1 to 20 hours. The surface-treatment may be carried out before or after the pulverization treatment. As a ball mill machine, there is used a rolling ball mill, a vibration ball mill, or a stirring mill such as a screw type stirring mill, a circulation tube type stirring mill, a stirring tank type stirring mill or an annular type stirring mill. The treatment is carried out in a batch treatment or a continuous treatment. Before or after the pulverization treatment, rough components are removed through a screen of preferably 100 to 500 meshes. As a high-purity quicklime, there is preferably used a quicklime having a silicon dioxide content of 0.2% by weight or less, particularly preferably 0.1% by weight or less, an alumina content of 0.04% by weight or less, particularly preferably 0.02% by weight or less, and a ferric oxide content of 0.02% by weight or less, particularly preferably 0.01% by weight or less.
The calcium hydroxide produced by any one of the above two methods can be surface-treated as follows. An aqueous solution in which an anionic surfactant in an amount of 0.1 to 10% by weight, preferably 0.5 to 5% by weight, based on the weight of the calcium hydroxide is dissolved, is added to the calcium hydroxide in a state where it is stirred and dispersed in water at a temperature (about 40xc2x0 C.) where the anionic surfactant is soluble or at a higher temperature, to surface-treat the calcium hydroxide. Thereafter, there can be carried out general treatments selected from filtration, washing with water, drying, pulverization, classification and the like as required. The surface-treatment is effective at dispersing the calcium hydroxide in a resin and at preventing the calcium hydroxide from converting to calcium carbonate due to carbonization. Therefore, the surface-treated calcium hydroxide is used. When calcium hydroxide is converted to calcium carbonate, activities such as acid-capturing properties are extremely decreased.
As a surface-treating agent used in the present invention, there is used an aqueous solution of an anionic surfactant or a phosphoric acid ester which works as anion in the aqueous solution. Examples of preferred surface-treating agents include alkali metal salts of saturated or unsaturated fatty acids having about five or more carbon atoms, such as sodium caprylate, sodium caprinate, sodium laurate, sodium stearate, sodium oleate, and sodium behenate; phosphoric acid esters of alkali metal salts or amine salts such as lauryl acid phosphate, oleyl acid phosphate and stearyle acid phosphate; sulfonates such as sodium alkylbenzene sulfonate and sodium alkyl sulfonate; and sulfates such as alkyl ether sulfate, alkyl aryl ether sulfate, alkylamide sulfate and alkyl sulfate. Of these, particularly preferred is an alkali metal salt of a higher fatty acid.
The resin used in the present invention includes the following examples. The examples include no-halogen-containing thermoplastic resins such as polyethylene, a copolymer of ethylene and other xcex1-olefin, a copolymer of ethylene and vinyl acetate, ethyl acrylate, or methyl acrylate, polypropylene, a copolymer of polypropylene and other xcex1-olefin, polybutene-1, polystyrene, a copolymer of styrene and acrylonitrile, a copolymer of ethylene and propylenediene rubber or butadiene, vinyl acetate, polyacrylate, polymethacrylate, polyurethane, polyester, polyether, and polyamide; chlorine-containing thermoplastic resins such as polyvinyl chloride, polyvinylidene chloride, chlorinated polyethylene, chlorinated polypropylene, a copolymer of vinyl chloride and vinyl acetate, a copolymer of vinyl chloride and ethylene, a copolymer of vinyl chloride and propylene, a copolymer of vinyl chloride and styrene, a copolymer of vinyl chloride and isobutylene, a copolymer of vinyl chloride and vinylidene chloride, a copolymer of vinyl chloride, styrene and maleic anhydride, a copolymer of vinyl chloride, styrene and acrylonitrile, a copolymer of vinyl chloride and butadiene, a copolymer of vinyl chloride and isoprene, a copolymer of vinyl chloride and chlorinated propylene, a copolymer of vinyl chloride, vinylidene chloride and vinyl acetate, a copolymer of vinyl chloride and an acrylic acid ester, a copolymer of vinyl chloride and a methacrylic acid ester, a copolymer of vinyl chloride and acrylonitrile, and a copolymer of vinyl chloride and any one of various vinyl ethers; halogen-containing rubbers such as fluorine-containing rubber, ethylene tetrafluoride, propylene rubber, chlorosulfonated polyethylene rubber, epichlorohydrin rubber, chloroprene rubber and brominated butyl rubber; and thermosetting resins such as a phenol resin, a melamine resin, an epoxy resin, an unsaturated polyester resin and an alkyd resin.
Of the above resins, particularly preferred are a halogen-containing resin such as polyvinyl chloride or polyvinylidene chloride and polyolefin such as polyethylene or polypropylene.
In the present invention, no special limitation is imposed upon the method of mixing and kneading the resin and the calcium hydroxide. Any mixing means may be adopted, so long as the means can uniformly mix both the components. For example, it includes a single-screw or two-screw extruder, a roll and a Banbury mixer. No special limitation is also imposed upon the molding method. Any known method may be adopted depending upon the kind of a resin and the kind of a desired molded article. For example, it includes an injection molding, an inflation film molding, a T-dice film molding, a calender molding, an extrusion molding, a blow molding, a press molding, a rotation molding, a sheet forming molding, a transfer molding, a laminate molding and a vacuum molding.
The calcium hydroxide-containing resin of the present invention may contain generally-used various additives other than the calcium hydroxide as desired. Examples of the additives include an antioxidant, an ultraviolet light absorber, a light stabilizer, a plasticizer, an antistatic agent, a pigment, a lubricant, a forming agent, a filler, a reinforcing agent, a crosslinker, a vulcanizing agent, a thermal stabilization assistant, and a processing stabilizer. The thermal stabilization assistant includes an organic acid salt of zinc such as zinc stearate, xcex2-diketone such as dibenzoyl methane and stearoyl benzoyl methane, polyhydric alcohols such as pentaerythritol, dipentaerythritol and trimethylol propane, perchlorates such as sodium perchlorate, perchlorate type hydrotalcite, CO3 type hydrotalcite, and phosphite.
The calcium hydroxide-containing resin of the present invention can be utilized in the following fields. When the resin is a halogen-containing resin such as polyvinyl chloride, the calcium hydroxide-containing resin of the present invention captures and neutralizes an acid substance such as hydrogen chloride occurring at a processing time or a burning (incinerating) time, whereby the calcium hydroxide-containing resin of the present invention can be used as a heat stabilizer for the resin, as an inhibitor of the occurrence of dioxin or as an inhibitor of deterioration of an incinerator due to an acid. For example, when the calcium hydroxide-containing resin of the present invention is used for a trash bag made of polyethylene or polypropylene or for a food-packing material, it serves to prevent dioxin from occurring by capturing an acid substance, such as a halogenated hydrogen, which occurs from garbage at a time of the incineration of the garbage. Further, owing to the incorporation of the calcium hydroxide-containing resin of the present invention in a food-packing material, antifungal properties appear so that the calcium hydroxide-containing resin of the present invention can be utilized for a food-packing material having the function of keeping freshness. When the calcium hydroxide-containing resin of the present invention is incorporated in a film for agriculture, for example, it can be utilized for preventing the deterioration of activity of HALS (hindered-amine-containing light stabilizer) due to an agricultural chemical. Further, it can be utilized for inactivating a residue of a Ziegler catalyst or a metallocene catalyst in polyolefin and utilized as a halogen-capturing agent. For example, it can be also utilized as an inhibitor of an acetic acid smell of a vinyl acetate-conataining resin. For example, it can be also utilized as a vulcanizing agent or a vulcanization promoter for a fluorine-containing rubber and a brominated butyl rubber. Furthermore, it can be also utilized as a thickening agent for FRP.